Field of the Invention
Petroleum as it is naturally produced from an underground formation, is in most cases a mechanical mixture of oil, entrained gas and salt water, some of which latter may be present as an oil/brine emulsion. It is desirable, and usually necessary to treat the petroleum thus produced at the wellhead, for the separation and removal of the entrained gas and emulsified brine, in order to render the oil pipelineable. Usually, the separated salt water is pumped back into the formation, in order to assist in maintaining the pressure therein, and also to resolve the salt water disposal problem. Separated gas is vented or flared, if in small quantities, and if in commercial volumes, is delivered to a pipeline for distribution. The equipment used for this three-phase separtion is known as a treater, and is generally quite familiar to those to whom the present invention will be addressed.
Such treaters normally involve the heating of the produced petroleum, in order to lower the viscosity of the fluid phase, and also to assist in the separation of the entrained gas. Brine droplets are coalesced either mechanically, as by forcing the emulsion through a series of perforated baffles; or electrostatically as by forcing the emulsion through a high-energy, electrically charged field; or chemically, by means of surface-active chemical agents which reduce the surface tension on the water droplets, thereby allowing them to coalesce into larger drops for separation by gravity. Frequently, two or more coalescing methods are employed in a treater.
Treaters have evolved in design from early developed open vats which maintained the produced petroleum in stationary condition for several days, permitting the entrained gas to freely separate to atmosphere and the salt water to separate to the bottom of the vat by gravity. There evolved heating methods in order to expedite the treatment by reducing the viscosity of the oil, as described. Subsequent development evolved the heater-treater which is the current state-of-the-art comprising an elongated enclosed tank having a burner-fired heater section and a downstream treater section for continuous flow, with a series of perforated baffles positioned within the treater section transversely to the flow of fluids; the perforated baffles function to promote the even distribution over the full cross-sectional area of the treater section of the fluids in motion, and to cause a pressure drop within the fluid across the perforated baffles which results in a release of entrained gases, which then collect in the upper volume of the tank for removal. However, salt water emulsions within the oil have continued to be inefficiently treated by gravity settling and baffling of the flow following heating; thus, further measures have been necessary in order to cause coalescing of the small droplets of brine into larger drops which could be settled out by gravity.
The conventional treatment has the operational disadvantages of being time-consuming, due to the residence-time required in the treater and the requirement that the petroleum be heated to a sufficiently high temperature to reduce the viscosity thereof so that coalescing of the emulsified droplets will be encouraged. The maintenance of a large quantity of oil at a relatively high temperature is costly of energy, and requires the equipment involved to be capable of sustained operation at the temperatures involved.
Treaters in current use are normally tanks in the form of elongated horizontal cylinders divided by means of internal partitions into compartments through which the petroleum will sequentially flow. Burner-fired heaters are normally included in the upstream heater section for heating the emulsion to the desired temperature, during which most of the entrained gas and some of the brine will separate from the emulsion. The partially-de-emulsified brine then flows into a treater section, in substantially gas-free state, encountering a series of baffles adapted to encourage even flow of fluids and to avoid the formation of flow channels within the fluid body, thereby to assist in separation of remaining gases and coalescing of water droplets, and their separation by gravity to the bottom of the tank for ultimate discharge removal.
Various techniques of improvement have heretofore been employed in order to minimize treatment time and heat energy consumption. In my U.S. Pat. No. 4,329,159, "Energy Saving Heavy Crude Oil Emulsion Treating Method and Apparatus for Use Therewith", there is described a method and apparatus of the type described, additionally including a number of metallic apertured grid electrodes suspended adjacent apertured baffles, the electrodes being supplied with electrical energy. A series of longitudinally spaced electrical fields of high potential are thereby created, which cause droplets of emusified brine to move in violent random fashion, the droplets coalescing and collecting into drops of sufficient weight as to fall by gravity to the lower portion of the treater section for removal.
While the addition of the electrostatic elements thus described has resulted in enhanced separation of the oil and brine, the invention now described discloses a further improvement, in which a series of coalescer or collector elements of unique design are positioned within the treater compartment each in immediate downstream relationship from an electrostatic element, the coalescer element being electrically grounded to the tank thereby causing the brine droplets to be attracted to the coalescer element where coalescing into brine drops occurs, followed by gravity collection and removal.
Even when electricity is not economically available, the present invention operates as a mechanical (non-electrolytic) treater in which a brine de-emulsifying element consists of a bundle of open-ended tubes having foreshortened vertical dimensions positioned downwardly inclined in the direction of flow. Individual tubes present opposition to the flow of emulsion for contact between the brine droplets of the emulsion and the interior surfaces of the tubes, the brine droplets adhering to the tube surfaces for accumulation and downwardly trickling by gravity to a sump for collection and removal. Each such tube having a diameter of approximately one inch and a length of approximately one foot, brine droplets moving forwardly with the emulsion and passing through the open-ended passages of the collector elements will be separated from the emulsion by contact with and adhesion to the interior surfaces of the tubes. Contact with a surface of one of such tubes will be facilitated in accordance with Stokes Law of Fluid Flow, as will be hereinafter described. Downward inclination of the tubes in the direction of flow assists in keeping the tubes clear of particulate solids which settle out from the oil, by permitting such solids to fall longitudinally through the tubes, for collection and removal.
In some cases of crude oil production where the oil is produced in association with salt water where the water content is extremely high (in some cases up to 90 percent), the non-emulsified water can readily be separated prior to the delivery of the emulsified oil and water to a treater. In such cases, a separator known a Free Water Knockout, incorporating the within invention, will function well by increasing the tube diameters and the flow velocities in order to accommodate the larger volumes of fluids.
In the method and apparatus of the present invention, an electrode grid is suspended in the emulsion upstream of the coalescer element to induce an electrostatic charge on the brine droplets as they move downstream into the region of the coalescer, which is electrically grounded to the tank. In addition to activating the random movements of the brine droplets, the electrostatic charge augments the attraction of the brine droplets to the coalescer, thus enhancing the separation of the emulsion.
A principal object of the present invention is to provide an apparatus and method for treating a heavy emulsion to obtain a substantially complete separation of brine and gas from the oil in acceptable time and with a lesser consumption of energy and the capability, but not the necessity, of using electrostatic elements.
Another object of the invention is to effect the coalescing of droplets of brine in the emulsion by subjecting the droplets to an electrostatic charge, causing coalescing thereof when electricity is available; then, regardless of the availability of electricity, causing such coalesced droplets to pass into a collector element in which the vertical component of their trajectory, before contacting a metallic surface of the coalescer element (the coalescer being electrically grounded when electricity is used), is effectively minimized, thereby achieving substantially complete separation of brine from the oil.
A yet more particular object of the invention is to provide an apparatus for separating oil, brine and gas from an emulsion, comprising a generally horizontal separating vessel including means for passing the emulsion through an electrical field then through a collector means in which flow paths having foreshortened vertical dimensions are provided, and which are forwardly and downwardly inclined, thereby providing a multiplicity of downwardly inclined brine collector surfaces on which the brine component of the emulsion may collect for trickling downwardly by gravity towards the bottom of the vessel for discharge.
These general objects as well as other and more specific objects of the invention will be fulfilled in the following description and claims, taken in conjunction with the attached drawings.